Multi-Axial Bone Screw Assembly

ABSTRACT

A bottom-loading multi-axial bone anchor apparatus is disclosed. The apparatus includes a receiver member, a crown member, a bone anchor and a retaining member. The receiver member defines an upper opening and a lower opening, which may form part of the same opening, a channel, and a groove. The crown member and bone anchor are loaded into the lower opening of the receiver member, and the retaining member fits around the bone anchor and into the groove in the receiver member. The bone anchor is capable of multi-axial positioning with respect to the receiver member. An elongated member is placed in the channel of the receiver member, contacting the crown member, and a compression member is applied via the upper opening. The compression member presses down on the elongated member, which presses down on the crown member and locks the bone anchor between the crown member and the retaining member.

FIELD OF THE INVENTION

The present invention relates to devices and implants used inosteosynthesis and other orthopedic surgical procedures. Specifically,the present invention contemplates a bottom loading bone anchor assemblycapable of achieving multiple angular orientations with respect to anelongated member extending along bone tissue.

BACKGROUND OF THE INVENTION

Several techniques and systems have been developed for correcting andstabilizing damage or malformation of bones, especially the long bonesand the spine. In one type of system, an elongated member such as abendable rod is disposed longitudinally along a length of the bone(s).In spinal applications, the rod is preferably bent to correspond to thenormal curvature of the spine in the particular region beinginstrumented. For example, the rod can be bent to form a normal kyphoticcurvature for the thoracic region of the spine, or a lordotic curvaturefor the lumbar region. In accordance with such a system, the rod isengaged to various vertebrae along a length of the spinal column by wayof a number of fixation elements. A variety of fixation elements can beprovided which are configured to engage specific portions of thevertebra and other bones. For instance, one such fixation element is ahook that is configured to engage the laminae of the vertebra. Anothervery prevalent fixation element is a screw that can be threaded intovarious parts of the vertebrae or other bones.

In one typical spinal procedure utilizing a bendable rod, the rod issituated on opposite sides of the spine or spinous processes. Aplurality of bone screws are threaded into a portion of severalvertebral bodies, very frequently into the pedicles of these vertebrae.The rods are affixed to these plurality of bone screws to applycorrective and stabilizing forces to the spine.

One example of a rod-type spinal fixation system is the TSRH® SpinalSystem sold by Medtronic Sofamor Danek, Inc. The TSRH® System includeselongated rods and a variety of hooks, screws and bolts all configuredto create a segmental construct throughout the spine. In one aspect ofthe TSRH® System, the spinal rod is connected to the various vertebralfixation elements by way of an eyebolt. In this configuration, thefixation elements are engaged to the spinal rod laterally adjacent tothe rod. In another aspect of the TSRH® System, a variable angle screwis engaged to the spinal rod by way of an eyebolt. The variable anglescrew allows pivoting of the bone screw in a single plane parallel tothe plane of the spinal rod. Details of this variable angle screw can befound in U.S. Pat. No. 5,261,909 to Sutterlin et al., owned by theAssignee of the present invention. One goal achieved by the TSRH® Systemis that the surgeon can apply vertebral fixation elements, such as aspinal hook or a bone screw, to the spine in appropriate anatomicpositions. The TSRH® System also allows the surgeon to easily engage abent spinal rod to each of the fixation elements for final tightening.

Another rod-type fixation system is the Cotrel-Dubosset/CD® SpinalSystem sold by Medtronic Sofamor Danek, Inc. Like the TSRH® System, theCD® System provides a variety of fixation elements for engagementbetween an elongated rod and the spine. In one aspect of the CD® System,the fixation elements themselves include a body that defines a slotwithin which the spinal rod is received. The slot includes a threadedbore into which a threaded plug is engaged to clamp the rod within thebody of the fixation element. The CD® System includes hooks and bonescrews with this “open-back” configuration. Details of this technologycan be found in U.S. Pat. No. 5,005,562 to Cotrel. One benefit of thisfeature of the CD® System is that the fixation element is positioneddirectly beneath the elongated rod. This helps reduce the overallbulkiness of the implant construct and minimizes the trauma tosurrounding tissue.

On the other hand, these fixation elements of the CD® System are capableonly of pivoting about the spinal rod to achieve variable angularpositions relative to the rod. While this limited range of relativeangular positioning is acceptable for many spinal pathologies, manyother cases require more creative orientation of a bone screw, forinstance, relative to a spinal rod. Certain aspects of this problem areaddressed by the variable angle screw of the TSRH® System, as discussedin the '909 patent. However, there is a need for a bone screw that iscapable of angular orientation in multiple planes relative to the spinalrod. Preferably, the bone screw is capable of various three-dimensionalorientations with respect to the spinal rod. Screws of this type havebeen referred to as poly-axial or multi-axial bone screws.

Others have approached the solution to this problem with variouspoly-axial screw designs. For example, in U.S. Pat. No. 5,466,237 toByrd et al., a bone screw is described which includes a sphericalprojection on the top of the bone screw. An externally threaded receivermember supports the bone screw and a spinal rod on top of the sphericalprojection. An outer nut is tightened onto the receiver member to pressthe spinal rod against the spherical projection to accommodate variousangular orientations of the bone screw relative to the rod. While thisparticular approach utilizes a minimum of components, the security ofthe fixation of the bone screw to the rod is lacking. In other words,the engagement or fixation between the small spherical projection on thebone screw and the spinal rod is readily disrupted when theinstrumentation is subjected to the high loads of the spine,particularly in the lumbar region.

In another approach shown in U.S. Pat. No. 4,946,458 to Harms et al., aspherical headed bone screw is supported within separate halves of areceiver member. The bottom of the halves are held together by aretaining ring. The top of the receiver halves are compressed about thebone screw by nuts threaded onto a threaded spinal rod. In anotherapproach taken by Harms et al., in U.S. Pat. No. 5,207,678, a receivermember is flexibly connected about a partially spherical head of a bonescrew. Conical nuts on opposite sides of the receiver member arethreaded onto a threaded rod passing through the receiver. As theconical nuts are threaded toward each other, the receiver memberflexibly compresses around the head of the bone screw to clamp the bonescrew in its variable angular position. One detriment of the systems inthe two Harms et al. patents is that the spinal rod must be threaded inorder to accept the compression nuts. It is known that threading rodscan tend to weaken the rods in the face of severe spinal loads.Moreover, the design of the bone screws in the '458 and '678 patentsrequire a multiplicity of parts and are fairly complicated to achievecomplete fixation of the bone screw.

A further approach illustrated in U.S. Pat. No. 5,797,911 to Sherman etal., owned by the Assignee of the present invention, is to provide aU-shaped holder through the top of which a bon fastener topped with acrown member is loaded. The holder accommodates a rod in a channel abovethe crown member and a compression member above the rod. The compressionmember presses on the rod and crown member to lock the fastener againstthe holder in any of a number of angles in three dimensions with respectto the rod. This approach has proven to be quite effective in addressingthe above-identified problems. However, it does not permitbottom-loading of the fastener. Additionally, the holder is somewhatbulky in order to accommodate the other structural components.

Yet a further approach is shown in U.S. Pat. No. 5,733,285 to Errico etal., in which a holder is provided with a tapered and colletted portionat the bottom into which a bone fastener head is inserted. A sleeve isprovided that slides down around the colletted portion to crush lock thecolletted portion around the head of the bone fastener. This apparatusis believed to be relatively bulky and difficult to manipulate given theexternal sliding locking mechanism. It is further dependent on the fitof the external sleeve and the relative strength of the collet and itsbending and crushing portions for secure locking of the bone fastenerhead.

There is therefore a need remaining in the industry for a multi-axialbone anchor that can be readily and securely engaged to an elongatedmember of any configuration i.e., smooth, roughened, knurled or eventhreaded—which achieves improved angulation of the bone anchor, improvedstrength, and reduced size, including profile and bulk, of thecomponents used to engage the bone anchor to the elongated member in anyof a variety of angular orientations.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a bone fixation assembly is providedthat includes a receiver member defining an upper opening portion and alower opening portion, a channel configured to receive an elongatedmember that communicates with the upper and lower opening portions, anda groove around a portion of the lower opening portion. The assemblyfurther includes a crown member with an upper surface and a lowersurface that is movably disposed in the lower opening portion, and abone-engaging anchor having a lower portion configured to engage a boneand a head smaller than the lower opening portion, with the head beingmovably disposed in the lower opening portion adjacent to the concavelower surface of the crown member. The assembly also includes aretaining member defining an aperture smaller than the head of the boneanchor that is at least partially housed in the groove of the receivermember and positioned around the bone anchor and below its head.

Additional embodiments, examples, advantages, and objects of the presentinvention will be apparent to those of ordinary skill in this art fromthe following specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of one embodiment of the multi-axialbone screw anchor assembly of the present invention.

FIG. 2 is an exploded view of the embodiment of the invention depictedin FIG. 1.

FIG. 3 a is a side elevational view of an embodiment of the receivermember of the embodiment of the invention illustrated in FIG. 2.

FIG. 3 b is a front elevational view of the embodiment of the receivermember illustrated in FIG. 3 a.

FIG. 3 c is a sectional view, taken along the lines 3 c-3 c in FIG. 3 a,and viewed in the direction of the arrows, of the embodiment of thereceiver member illustrated in FIG. 3 a.

FIG. 3 d is a sectional view, taken along the lines 3 d-3 d of FIG. 3 band viewed in the direction of the arrows, of the embodiment of thereceiver member illustrated in FIG. 3 a.

FIG. 4 a is a side elevational view of an embodiment of a bone anchorused in the embodiment of the invention illustrated in FIG. 2.

FIG. 4 b is a sectional view, taken along the lines 4 b-4 b of FIG. 4 aand viewed in the direction of the arrows, of the embodiment of the boneanchor illustrated in FIG. 4 a.

FIG. 4 c is a magnified view of one embodiment of the head of theembodiment of the bone anchor illustrated in FIG. 4 a.

FIG. 5 a is a top view of one embodiment of a crown member used in theembodiment of the present invention illustrated in FIG. 2.

FIG. 5 b is a sectional view, taken along the lines 5 b-5 b in FIG. 5 aand viewed in the direction of the arrows, of the embodiment of thecrown member illustrated in FIG. 5 a.

FIG. 5 c is a sectional view substantially similar to FIG. 5 b ofanother embodiment of a crown member used in the embodiment of theinvention illustrated in FIG. 2.

FIG. 6 a is a top view of one embodiment of a retaining member used inthe embodiment of the invention illustrated in FIG. 2.

FIG. 6 b is a sectional view, taken along the lines of 6 b-6 b in FIG. 6a and viewed in the direction of the arrows, of the embodiment of theretaining member illustrated in FIG. 6 a.

FIG. 7 is an enlarged sectional view of the embodiment of the presentinvention illustrated in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the principles of theinvention, reference will now be made to the embodiment illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of theinvention is thereby intended, such alterations and furthermodifications in the illustrated device, and such further applicationsof the principles of the invention as illustrated therein, beingcontemplated as would normally occur to one skilled in the art to whichthe invention relates.

Referring generally to FIGS. 1 and 2, there is shown one embodiment of amulti-axial bone anchor assembly 20 of the present invention. In theillustrated embodiment, assembly 20 includes a receiver member 30, abone anchor 50, a crown member 70, and a retaining member 90. Theassembly 20 of the present invention is designed for use with anelongated member R (FIG. 7) such as a spinal rod, bar or otherorthopedic construct, as further described below.

Referring now generally to FIGS. 3 a-3 d, one embodiment of the receivermember 30 of the present invention is shown. Receiver member 30 definesan upper opening portion 31 a and a lower opening portion 31 b, which inthe illustrated embodiment form a single opening 32 extending throughreceiver member 30 from an upper aperture 33 in top end 34 to a loweraperture 35 in bottom end 36. Lower opening portion 31 b of opening 32,in one specific embodiment, includes a chamber 38 defined by a chamberwall 39. Alternatively, upper and lower opening portions 31 a, 31 b canhave a variety of configurations, such as each having one or moresections of differing diameter.

Opening 32 is partially surrounded by a chamfered or rounded edge 40 aat top end 34 of receiver member 30, and is surrounded by chamfered orrounded edge 40 b at the bottom end 36 of receiver member 30. Proximateto bottom end 36, receiver member 30 defines a groove 41 and associatedledge 41 a around opening 32. In the illustrated embodiment, groove 41extends around the entire perimeter of opening 32, although it will beseen that groove 41 could extend only partially around the perimeter ofopening 32. Groove 41 has a groove depth A (FIG. 7) and a groovediameter B (FIG. 3 a).

Receiver member 30 in the illustrated embodiment includes a pair ofupright branches 42, 43 through which opening 32 extends. Branches 42,43 further define a U-shaped channel 45 transverse to opening 32 thatcommunicates with upper portion 31 a and lower portion 31 b of opening32, and that accommodates an elongated member R (FIG. 7). In a specificembodiment, internal threads 44 are formed in branches 42, 43, andbranches 42, 43 are provided with indentations or holes 46, which allowthe surgeon to grip receiver member 30 with an appropriate tool (notshown). Internal thread 44 in a specific embodiment is a reverse anglethread, i.e. a thread in which the forward face points down and intoward receiver member 30, as disclosed in commonly-owned U.S. patentapplication Ser. No. 09/188,825, filed Nov. 9, 1998, the disclosure ofwhich is hereby incorporated by reference. Preferably, the top portion47 of receiver member 30 (which includes branches 42, 43) is narrowerthan bottom portion 48 of receiver member 30, thereby reducing the bulkand profile of receiver member 30.

Referring now generally to FIGS. 4 a-4 c, an embodiment of a bone anchor50 used in the present invention is shown. The illustrated bone anchor50 is a bone screw, which in one embodiment is substantially like thebone screw disclosed in U.S. Pat. No. 5,885,286, the disclosure of whichpatent is hereby incorporated by reference. Bone anchor 50 includes ananchorage portion 52 and a head portion 54. Anchorage portion 52includes at least one thread 56, which may be a cancellous self-tappingthread. Head portion 54 forms part of a sphere in the illustratedembodiment, though alternative curvate and other configurations may beemployed. Head 54 in one particular embodiment includes a series ofridges 58 for improving purchase with the inside of crown member 70(described below). Head 54 may have alternative friction-increasingsurface configuration(s) such as roughening or knurling. Further, head54 includes a tool-engaging print 60, with which a tool (not shown) maybe engaged to drive anchorage portion 52 into a bone. Tool-engagingprint 60 is an interior print in the illustrated embodiment, although anexterior print could be used, and it may have any of a number ofconfigurations, such as hexagonal, hexalobate, or other knowntorque-transferring configurations.

Other embodiments of bone anchor 50 are contemplated as being within thescope of the present invention. For example, bone anchor 50 could be abone-engaging hook rather than a screw. In that embodiment, anchorageportion 52 would be configured with a hook rather than an elongatedsection with thread 56.

Head 54 of bon anchor 50 is shaped and sized to fit within at leastlower portion 31 b of opening 32 and chamber 38 of receiver member 30.Specifically, head 54 has a width that is smaller than the width oflower opening portion 31 b and chamber 38. As more fully describedbelow, bone anchor 50 is inserted into receiver member 30, with head 54entering lower opening portion 31 b and chamber 38 through bottom end 36of receiver member 30.

Referring now to FIGS. 5 a-5 b, there is shown one embodiment of crownmember 70 of the present invention. In that embodiment, crown member 70is in the shape of a circular disc, having an upper surface 72 with abeveled edge 74 and a lower surface 78. Lower surface 78 is configuredto accommodate head 54 of bone anchor 50, and therefore the illustratedembodiment of lower surface 78 has the shape of part of a sphere.Alternatively or additionally, the lower surface of crown member 70 canhave one or more other shapes, such as beveled or conical lower surface78′ (FIG. 5 c). Lower surface 78 can be provided with a friction- orpurchase-enhancing surface configuration (e.g. roughening or knurling)for cooperation with head 54 of bone anchor 50.

The illustrated embodiment of crown member 70 also includes a hole 80.Hole 80 is provided so that head 54, and specifically tool-engagingprint 60, of bone anchor 50 may be accessed through crown member 70.Crown member 70 is sized and shaped to fit within at least lower portion31 b of opening 32 and chamber 38 of receiver member 30. The outerdimension of crown member 70 is preferably slightly smaller than theinner dimension of chamber 38 and lower portion 31 b of opening 32 sothat crown member 70 is slidably and rotatably movable within chamber 38and opening 32. Further, in the illustrated embodiment the outerdimension of crown member 70 is larger than the inner dimension of upperopening portion 31 a, so that crown member 70 cannot move into upperopening portion 31 a.

Referring now to FIGS. 6 a-6 b, there is shown one embodiment ofretaining member 90 of the present invention. In the illustratedembodiment, retaining member 90 has the form of a C-shaped spring orclip defining a gap 91. Retaining member 90 includes a top surface 92and a bottom surface 94. In the illustrated embodiment, retaining member90 also includes internal surfaces 96, 98, 100 that substantiallysurround aperture 102. In one specific embodiment, internal surface 96forms a portion of a sphere of radius substantially identical to theradius of head 54 of bone anchor 50, internal surface 98 is cylindrical,and internal surface 100 is conical and angled outward to allow agreater range of angular positioning of bone anchor 50. In alternativeembodiments, there may be single or multiple internal surfacessurrounding aperture 102, which surface(s) may be cylindrical, conical,spherical or of other appropriate configuration. The diameter ofaperture 102 is smaller than the diameter of head 54 of bone anchor 50and the diameter of crown member 70.

Retaining member 90 has an unloaded or natural outer diameter D, i.e. adiameter measured when retaining member 90 is under no contractive(gap-closing) or expansive (gap-opening) stress. Diameter D of retainingmember 90, in one embodiment, is less than groove diameter B of groove41. Further, retaining member 90 has a body width W that issubstantially constant throughout retaining member 90. Body width W ofretaining member 90 is greater than groove depth A of groove 41.

Generally referring to FIGS. 1, 2 and 7, assembly 20 is assembled asfollows: bone anchor 50, crown member 70 and retaining member 90 areinserted into receiver member 30 through bottom end 36, eitherindividually or substantially in one step. For example, crown member 70may be inserted first, followed by bone anchor 50 with retaining member90 being inserted last. In one specific embodiment, retaining member 90is fitted around bone anchor 50 just below head 54 prior to insertion ofbone anchor 50 into receiver member 30. Retaining member 90 can beplaced around bone anchor 50 by inserting anchorage portion 52 of boneanchor 50 through aperture 102 of retaining member 90 and movingretaining member 90 over anchorage portion 52 toward head 54.Alternatively, gap 91 of retaining member 90 may be pressed against theshank of bone anchor 50 below head 54, so that gap 91 expands to allowplacement of bone anchor 50 within aperture 102 of retaining member 90,whereupon retaining member 90 returns to its original size and shape. Byplacing crown member 70 atop head 54 of bone anchor 50, so that lowersurface 78 of crown member 70 adjoins head 54, and fitting bone anchor50 and retaining member 90 together as described above, simultaneousinsertion of bone anchor 50, crown member 70 and retaining member 90into receiver member 30 can be accomplished.

Crown member 70 remains slideably and rotatably positioned in lowerportion 31 b of opening 32 and/or chamber 38 of receiving member 30, andbone anchor 50 remains multi-axially moveable with respect to crownmember 70 and receiving member 30. Retaining member 90 is forced upwardinto lower portion 31 b of opening 32. Retaining member 90 contracts,making gap 91 smaller, as retaining member 90 is forced againstchamfered edge 40 b of receiving member 30, until the outer diameter ofretaining member 90 is the same as the diameter of lower portion 31 b ofopening 32. Retaining member 90 is further advanced along opening 32 andinto groove 41 so that retaining member 90 is fitted into at least aportion of groove 41.

As noted above, in one specific embodiment the groove diameter B ofgroove 41 is smaller than the outer diameter D of retaining member 90 inits natural (i.e., unloaded) condition. Thus, when retaining member 90is within groove 41, retaining member 90 presses against the walls ofgroove 41. Alternatively, groove diameter B of groove 41 may be the samesize or slightly larger than the natural outer diameter D of retainingmember 90. In this case, the lower surface 94 of retaining member 90rests upon ledge 41 a of groove 41, and thereby holds retaining member90 within groove 41. Groove depth A of groove 41 is less than the bodywidth W of retaining member 90, so that when retaining member 90 isfitted in groove 41, a portion of retaining member 90 projects intolower opening portion 31 b of opening 32.

When retaining ring 90 is seated within groove 41, bone anchor 50 andcrown member 70 are retained within opening 32 of receiver member 30.Crown member 70 is supported by head 54 of bone anchor 50, and head 54is supported by internal surface 96 of retaining member 90. Retainingmember 90 is held by groove 41 and/or ledge 41 a of receiver member 30,and thus bone anchor 50 and crown member 70 will not pass throughretaining ring 90 and out of receiver member 30 when retaining ring 90is within groove 41.

Preferably, assembly 20 is assembled (as described above) prior to usein a surgical procedure. In using the illustrated embodiment of assembly20, bone anchor 50 of assembly 20 is threaded into an appropriatelyprepared hole in a bone (not shown). It will be understood that inalternative embodiments of the invention, for example where bone anchor50 is a bone hook, drilling a hole in bone and threading the anchortherein may not be necessary. Threaded anchoring portion 52 is insertedinto the hole, and an appropriate screwing tool is used withtool-engaging print 60 of bone anchor 50 through hole 80 in crown member70, and bone anchor 50 is threaded into the bone. When bone anchor 50has been threaded into the bone to the desired depth, receiver member 30is positioned so that opening 32 forms a desired angle with bone anchor50, as depicted in FIG. 1. In the illustrated embodiment, the angle θbetween bone anchor 50 and opening 32 can be any value up to 30 degreesin any direction. It will be seen that the maximum angle of bone anchor50 relative to opening 32 can be changed in several ways, for example bythinning the portion of bone anchor 50 beneath head 54, by providingsteeper angulation of chamfered edge 40 b, and/or by placing groove 41as close as possible to bottom end of 36 of receiver member 30.

As described above, receiver member 30 may be angled as the surgeondesires with respect to bone anchor 50. An elongated member R such as aspinal rod, connector, or other orthopedic surgical implant is coupledwith assembly 20. Elongated member R is placed in channel 45 of receivermember 30, and contacts top surface 72 of crown member 70. A compressionmember 120, such as a set screw or threaded plug, is threaded intothreads 44 of receiver member 30 and down onto elongated member R.Compression member 120, in one embodiment, is a set screw or plug havingexternal threads 122 and a print 124 for applying torque, and in aspecific embodiment is a break-off set screw as disclosed in U.S. Pat.No. 5,885,286 to Sherman et al., incorporated herein by reference. In afurther embodiment, thread 122 is a reverse angle thread as disclosed inU.S. patent application Ser. No. 09/188,825, filed Nov. 9, 1998,incorporated herein by reference, which is compatible with the reverseangle embodiment of thread 44 of receiver member 30, described above.Alternatively, where receiver member 30 is externally threaded,compression member 120 could be an internally-threaded nut.

As compression member 120 is tightened, elongated member R is forceddownward against crown member 70, which pushes crown member 70 down ontohead 54 of bone anchor 50. Head 54 is thereby clamped between retainingmember 90 and crown member 70. In the embodiment of the invention inwhich head 54 includes ridges 58, ridges 58 are pressed into lowersurface 78 of crown member 70. In this way, bone anchor 50 is lockedinto the desired angular position with respect to elongated member R andthe remainder of assembly 20.

Alternatively, assembly 20 can be assembled during the surgicalprocedure. Bone anchor 50, with retaining ring 90 already positionedbeneath head 54, is inserted into the bone. Crown member 70 is placedatop bone anchor 50 or in opening 32 in receiver member 30. Receivermember 30 is then pressed down onto head 54 of bone anchor 50, forcingretaining ring 90 to contract, to enter opening 32, and to seat ingroove 41 as described above. After assembly 20 is assembled in thisfashion, an elongated member is loaded into receiver member 30 andlocked as previously described.

Preferred materials for the present invention include stainless steeland titanium. It will be recognized that any sturdy biocompatiblematerial may be used to accomplish the osteosynthesis and otherorthopedic surgical goals of the present invention. In one specificembodiment, crown member 70 may be made of a material somewhat softerthan the material used for ridges 58 of head 54 of bon anchor 50. Suchconstruction will allow ridges 58 to penetrate somewhat more easily intointerior surface 78 of crown member 70 during locking of assembly 20,thereby providing a more definite purchase between ridges 58 and crownmember 70. In another specific embodiment, crown member 70 may be madeof a material somewhat softer than the material used for elongatedmember R. Such construction will allow upper surface 72 of crown member70 to deform to the shape of elongated member R during locking ofassembly 20, also providing a more secure locking of the implant.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, the same is to be considered asillustrative and not restrictive in character, it being understood thatonly the preferred embodiment has been shown and described and that allchanges and modifications that come within the spirit of the inventionare desired to be protected.

1-43. (canceled)
 44. A method, comprising: providing a receiver memberdefining an upper opening portion and a lower opening portion, a channeltransverse to and communicating with said upper opening portion and saidlower opening portion, and a groove around at least a portion of saidlower opening portion; providing a retaining member having an innerdimension and an outer dimension; providing a bone anchor member havinga head at least partially within said lower opening portion; insertingsaid head of said bone anchor member into said lower opening so that aportion of said head is substantially above said groove; and insertingsaid retaining member into said lower opening so that at least a portionof said retaining member is within said groove.
 45. The method of claim59, further comprising positioning said retaining member around aportion of said bone anchor member.
 46. The method of claim 60, whereinsaid positioning step is performed prior to both of said insertingsteps.
 47. The method of claim 60, further comprising providing a crownmember and inserting said crown member in said receiver member.